1. Field of the Invention
This invention relates to plasma-etching apparatus, and more particularly to a plasma etching apparatus which can etch the surface of a work piece uniformly over its entire area.
2. Description of the Prior Art
As is well known, the term "plasma etching" signifies a process wherein, e.g., a halide such as CF.sub.4, CCl.sub.4, and BCl.sub.3 or a gas such as O.sub.2, N.sub.2, Ar, and air is introduced into a reaction tube to establish a predetermined pressure, radio-frequency power is applied across electrodes to generate a plasma, and the surface of a work piece placed in the reaction tube is etched by radicals produced by the plasma.
Equipment for use in such plasma etching is classified into the induction coil type and the capacitance type on the basis of the configurations of the electrodes for generating the plasma. The structure of a typical plasma etching apparatus of the capacitance type in the prior art is shown in FIG. 1. This apparatus is also called the "barrel" type. Semi-annular electrodes 1 and 2 are opposingly arranged on the outer wall of a reaction tube 4 with a desired gas introduced therein, and are respectively connected to a radio-frequency power source. A work piece 3 to be etched is placed substantially in the central part of the interior of the reaction tube 4.
With the prior art apparatus having the electrodes of such shape, in the case where CF.sub.4, O.sub.2, N.sub.2, Ar, air or the like is employed as the gas to be introduced into the reaction tube, the plasma is comparatively homogeneously distributed in the reaction tube, so that the etching proceeds substantially uniformly without a very serious hindrance in practical use. However, when it is intended to perform the etching by introducing a gas containing chlorine, such as CCl.sub.4 and BCl.sub.3, the plasma forms only in regions where the radio-frequency potential is high. Therefore, the etching of the work piece placed in the reaction tube becomes non-uniform.
In order to solve this problem, there has been proposed a plasma-etching apparatus of the structure shown in FIG. 2. As apparent from the figure, this apparatus is constructed such that electrodes 11 and 12 formed as two parallel plates are disposed within a reaction tube 4 and a work piece 3 is situated between the parallel plate electrodes 11 and 12. This construction is sometimes called the "planar" type.
The plasma-etching equipment employing the parallel plate electrodes can generate homogeneous plasma across the electrodes 11 and 12 even when the etching is carried out by introducing a gas containing chlorine, such as CCl.sub.4 and BCl.sub.3. However, in the case where the work piece 3 is made of an insulator or where although the work piece is made of a conductor it is insulated from the electrodes, the plasma becomes inhomogeneous in such a manner that the intensity is higher in the peripheral part than the central part of the work piece. For this reason, especially the edge part of the work piece and the vicinity thereof is conspicuously etched as compared with the central part, and it is impossible to uniformly etch the whole surface of the work piece.
It has accordingly been difficult to manufacture by the plasma-etching process any component requiring etching of particularly high precision, for example, a photomask substrate for fabricating a semiconductor device.
The plasma-etching equipment of the "barrel" type and the "planar" type have been extensively used, and are described in many publications, for example, Kodak Microelectronics Seminar Proceedings, Interface '77, pages 55-65.
In order to reduce the non-uniformity of the etching, there has been proposed a method wherein a planar-type apparatus is employed, and the etching is executed while rotating an electrode on which a work piece is put (Kodak Microelectronics Seminar Proceedings, Interface '77, pages 33-41). However, it has been difficult with such prior art equipment and methods to achieve a uniform etching satisfactory in practical use, and superior plasma-etching apparatus and method has been desired.